1. Field of the Invention
The present invention relates to an over sampling analog-to-digital converter, and more particularly, to a high-order delta-sigma modulator.
2. Description of the Related Art
Analog-to-digital converters include Nyquist analog-to-digital converters and over sampling analog-to-digital converters. For measurement in a relatively narrow signal band, there has been used an over sampling analog-to-digital converter, a precision of which is easily increased and which has a small circuit scale. In particular, a delta-sigma modulator, which is a type of the over sampling analog-to-digital converter, has often been used.
The analog-to-digital converter includes a delta-sigma modulator including: amplifying/integrating circuits in a plurality of stages, each of which is configured to amplify and integrate a differential signal between an input analog signal and an expected feedback analog signal; a quantizer configured to convert an output of the amplifying/integrating circuit into a digital value; and a digital/analog converter configured to generate the feedback analog signal with the digital value, and a digital filter that includes, for example, a decimation filter and is configured to calculate a final analog/digital converted value based on a numerical sequence of digital values output from the delta-sigma modulator.
The precision of the analog-to-digital converter employing this system depends on the configuration of a delta-sigma modulator, and hence, in order to realize the high precision, a delta-sigma modulator is used that includes amplifying/integrating circuits connected in series in a plurality of stages. As the number of stages of the amplifying/integrating circuits is increased, the precision is improved and the circuit scale and the power consumption are increased.
In terms of the circuit scale and the power consumption, the number of stages needs to be small, but the number of stages of the differential amplifying/integrating circuits needs to be increased for the higher precision. Thus, a delta-sigma modulator having a small circuit scale is needed.
FIG. 8 is an illustration of an example of a delta-sigma modulator of related-art delta-sigma modulators.
The related-art delta-sigma modulator is a third-order delta-sigma modulator including a differential amplifying/integrating circuit 200, amplifying/integrating circuits 201 and 202, and a quantizer 203.
The differential amplifying/integrating circuit 200 in a first stage includes an amplifier configured to amplify (b times) an input signal Vin, a digital/analog converter 104 configured to convert an output Dout of the quantizer 203 into an analog signal, an amplifier configured to amplify (−b times) an analog signal, an adder circuit configured to add together output signals of the two amplifiers, and an integrating circuit 100 configured to integrate an output of the adder circuit.
The amplifying/integrating circuit 201 in a second stage includes an amplifier configured to amplify (c1 times) an output of the differential amplifying/integrating circuit 200, and an integrating circuit 101 configured to integrate an output of the amplifier.
The amplifying/integrating circuit 202 in a third stage includes an amplifier configured to amplify (c2 times) an output of the amplifying/integrating circuit 201 in the second stage, and an integrating circuit 102 configured to integrate an output of the amplifier.
The quantizer 203 includes an adder configured to add together the input signal Vin, an amplified (a1 times) signal of an output of the differential amplifying/integrating circuit 200 in the first stage, an amplified (a2 times) signal of an output of the amplifying/integrating circuit 201 in the second stage, and an amplified (a3 times) signal of an output of the amplifying/integrating circuit 202 in the third stage, and a comparator 103 configured to compare the added signal to an expected reference voltage.
A signal transfer function STF(z) and a noise transfer function NTF(z) of the related-art third-order delta-sigma modulator are represented by the following expressions, respectively.STF(z)=1NTF(z)=(z−1)3/{(z−1)3+b·a1·(z−1)2+b·a2·c1·(z−1)+b·a3·c2·c1}
A signal component is passed through as it is. However, delay integrating circuits are provided in the three stages, and hence a quantization noise is shifted to a high frequency side with the third-order characteristic of z. Note that, the signals are transferred with three clock delay.
FIG. 9 is a circuit diagram for illustrating an example of the related-art third-order delta-sigma modulator. In the example of FIG. 9, input signals are differential signals (Vin+ and Vin−).
The related-art third-order delta-sigma modulator includes switched capacitor amplifiers 300, 301, and 302, and a quantizer 303. The switched capacitor amplifiers 300, 301, and 302 can integrally realize an amplifying function and an integrating function. The quantizer 303 includes a capacitor configured to sample a signal or a reference signal, and a comparator 305 configured to compare a signal input thereto via the capacitor to an expected reference voltage. Each of the switched capacitor amplifiers 300, 301, and 302 performs (a) signal sampling/previous signal holding operation and (b) amplifying/integrating operation at the same timing.
FIG. 10 is a functional diagram of a related-art second-order delta-sigma modulator including amplifying/integrating circuits in two stages. A signal transfer function STF(z) and a noise transfer function NTF(z) of the second-order delta-sigma modulator are represented by the following expressions, respectively.STF(z)=1NTF(z)=(z−1)2/{(z−1)2+b·a1·(z−1)+b·a2·c1}
The signal transfer function STF(z) and an exhibited characteristic for a signal component are the same as those of the third-order delta-sigma modulator. The number of amplifying/integrating circuits is smaller than the third-order delta-sigma modulator by one stage, and hence a quantization noise is shifted to a high frequency side with the second-order characteristic of z.
However, in the related-art delta-sigma modulator, the number of differential amplifiers, which are necessary for holding and amplifying/integrating signals, needs to be the same as that of stages of the switched capacitor amplifiers. That is, three differential amplifiers are needed in the third-order delta-sigma modulator, and two differential amplifiers are needed in the second-order delta-sigma modulator.
The related-art delta-sigma modulator needs as many differential amplifiers as stages of the amplifying/integrating circuits, and hence it is difficult to reduce the circuit scale and the power consumption.